Electrolysis of sodium sulfate



May 26, 1964 J. KERTI 3,134,729

ELECTROLYSIS OF SODIUM SULFATE Filed Sept. 29, 1961 Fig.3

. JOZ'SEF KERT/ INVENTOR.

AGENT United States Patent Muse The invention concerns a process for the electrolytic conversion of sodium sulphate to sulphuric acid and caustic soda solution with the use of a mercury electrode.

In the electrolysis with a mercury cathode of an aqueous sodium sulphate solution, sulphuric acid is liberated on the anode, which must be separated from the mercuryelectrode cathode by means of a diaphragm. In such processes, the current yield is too low because of the participation of mobile hydrogen and hydroxyl ions in the circuit, and consequently only sulphuric acid and alkali diluted by sodium sulphate, can be produced. It has been proposed to use during the electrolysis a rotating anode in order to separate the evolved sulphuric acid, which is adherent to the surface of the anode, from the sodium sulphate solution. All these processes operate with a low current yield, are relatively costly and require complicated apparatus so that the electrolytic decomposition of sodium sulphate has not proved successful in practice. In very many processes used in heavy industry, for example, in the viscose process, sodium sulphate is formed as a by-product. Consequently, an economical conversion of this by-product to more valuable materials is highly desirable.

The invention relates to a process for working-up sodium sulphate to sulphuric acid and caustic soda with the use of a mercury electrode, in which the sulphuric acid and the caustic soda solution are obtained in relatively high concentration and purity, as well as with a good current yield, so that this process can be carried out readily and economically on a large scale.

In the process, lead and mercury electrodes are used, the lead electrode serving as the anode and the sulphate ions being separated from the sodium sulphate-containing electrolyte on the lead anode, as a result of which the lead electrode is converted into lead sulphate, and sodium amalgam is formed on the mercury electrode, from which, by decomposition in a separate cell, sodium hydroxide is produced. Then the sodium sulphate solution is replaced in the electrolytic cell by sulphuric acid solution, and the previously formed lead sulphate, operated as a cathode, is reduced to lead, Whilst the sulphuric acid concentration of the solution is increased. According to the invention, there is used in this process for the electrolysis a cell, in which between the lower lead electrode and upper mercury electrode an indifferent electrode is disposed and during the electrolysis of the sodium sulphate solution sufiicient solution is used that the mercury electrode, serving as the cathode, is covered and only suilicient solution is used for the formation of the sulphuric acid that the solution comes into contact only with the indifferent electrode which electrode is operated in the formation of the sulphuric acid as an anode.

The mercury electrode is disposed in an iron channel provided preferably with an insulating coating, through which the mercury flows, advantageously in such a manner that the mercury is passed from the cell in which the sodium sulphate is electrolyzed into a compartment, in which the sodium amalgam is decomposed and then returned to the electrolytic cell. In this way it is possible to maintain the sodium content of the sodium amalgam at a low value, practically below 0.1%. With this concentration, a sodium amalgam formation with a very good current yield can be obtained.

The electrolyte cells are preferably counter-currently connected in series, and the sodium sulphate solution is successively passed through several cells, whereby the sodium sulphate solution dilutes. Also the sulphuric acid solution can be passed through several cells during which the concentration of the sulphuric acid increases. Thus, caustic soda solution of 70% concentration and sulphuric acid of 30-40% concentration can practically be produced with a very good current yield, the sodium sulphate contamination of the products being very low.

The electrolysis is preferably carried out with a concentrated sodium sulphate solution, advantageously between 40-70" C.

As the lead electrode, there is preferably used a porous lead powder electrode and an adhesion-preventing material, for example a small quantity of barium sulphate, is admixed with the lead grains, in order to prevent a sintering of the grains. In the lead sulphation, one works preferably with a current density of l to 12 amperes calculated for a basic area of l dm. If the current density is too low, the lead is sulphated with a poor yield and lead hydroxide is formed; in contradistinction lead dioxide is formed with a high current density. The geometric surface of the lead electrode is preferably 2 to 10 times larger than that of the mercury electrode.

An embodiment of the invention is illustrated with reference to the accompanying drawings by way of example, in which:

FIG. 1 shows the cross-section of a cell,

FIG. 2 shows a group of series-connected cells, and

FIG. 3 shows the plan view of the group of cells illustrated in FIG. 2, in the open condition.

A cell vessel l is made of sheet iron coated with hard rubber 2, on the bottom of which is disposed an active lead powder layer 3, which contains 1% of barium sulphate dust. A glass fabric 4 which is secured in position by a synthetic resin mesh 5, is supported in the lead powder layer 3. The mercury 7 is located in a channel 6 coated with an insulating synthetic resin. Beneath the channel 6, there is located the electrode made of lead and containing 1% of silver, the part of which facing the lead powder is ribbed, in order to form a large surface. A glass fabric 9 extends between the electrode 8 and the channel 6.

When electrolyzing sodium sulphate, t e cell is completely filled with the electrolysis solution so that channel 6 and mercury 7 are fully covered by the-solution. During the electrolysis, the lead anode 3 is sulphated and sodium amalgam forms on the mercury cathode 7. The concentrated sodium sulphate solution is passed, for example, into cell A, from where the solution flows into cell B and so on into cell F, and the sodium content of the solution is reduced because of the electrolysis. In

' cells G-M, the lead electrode 3 is operated as a cathode, whilst the indifferent electrode S is operated as anode. The level of the electrolyte in the cell lies beneath the glass fabric 9. The glass fabric is used to prevent contamination of the mercury electrode with sulphuric acid drops during the electrolysis. The cells GM are filled with sulphuric acid containing electrolyte and the lead electrode is operated as cathode, whilst the indifferent electrode 8 is operated as anode. During the electrolysis, the lead sulphate on the sulphated electrode 3 is reduced to lead, while sulphuric acid is formed. Oxygen is formed at the electrode 8 and is led off from the closed cell through a line It). Sulphuric acid is formed in the cells G-M. The contents of cell G is allowed to flow oil" into cell H, and dilute sodium sulphate solution is then led from the cell F into the empty cell. The content of cell A is passed into cell B, whereupon a concentrated sulphuric acid solution is fed from the cell M into cell A. When changing the electrolyte, also the level of the electrolyte is changed and when using sodium sulphate, the mercury electrode 7 is operated as cathode. When the content of cell G is passed into cell H, concentrated sulphuric acid solution is obtained at the same time from cell M and a more dilute sulphuric acid solution is fed from the cell L into cell M, whereupon this solution is fed into cell A, during which time the sulphuric acid formation in cells H-I-JKL- M-A and the sodium sulphate electrolysis in the cells B-C-DE--F-G are carried out. The process is continued progressively in the manner described with each cell.

It can be seen from FIGURE 3 that the channel 6 passes over into the decomposition cell 12, returns from there into the electrolytic cell and twists further along the electrolytic and decomposition cells. It thus results that the sodium concentration of the amalgam in the electrolytic cells remains low since the amalgam is in the meantime always decomposed in the decomposition ccli. The solution is passed counter-currently into the series of decomposition cells 12 so that the concentration of sodium hydroxide is increased stepwise. One proceeds also here with each decomposition cell at the same speed, as in the case of the cells in which the sodium sulphate is electrolyzed, and at the same speed as the temporary draining of the dilute sodium sulphate solution, concentrated sodium hydroxide solution is here obtained successively from the individual decomposition cells.

In the course of the process, the concentrated sodium sulphate solution arrives in the cell in which the sulphation has progressed to the greatest extent, whilst the dilute sodium sulphate solution arrives in the cell in which a fresh lead surface is still available. The dilute sulphuric acid is fed into that cell in which the largest part of the lead sulphate is reduced, thereagainst the most concentrated sulphuric acid is further concentrated in the most sulphated cell. The concentrated sulphuric acid is fed into each cell from which previously a concentrated sodium sulphate solution has been drained; in this manner, the sulphuric acid takes up a certain quantity of sodium sulphate. In most cases the Na SO contamination of the sulphuric acid is not at all disturbing. In order to avoid it, the cell is preferably rinsed beforehand with sulphuric acid solution. The number of the series-connected sulphuric acid-containing cells is larger than that of the sodium sulphate decomposition cells, since with a formation of sulphuric acid the current yield is lower by several percent than in the case of sodium separation for alkali formation, and thus, if one worked with the same number of cells, in the working method using series-connected cells, several percent of lead sulphate would remain over after each cycle so that the quantity of the lead sulphate would be increased stepwise and the possibility of new sulphation thus decreased The lead cathode is preferably made of lead powder containing about 1% of barium sulphate, which is used in a quantity of 350 g./dm. The distance of the lead electrode to be sulphated from the level of the mercury is advantageously 2-3 cm. When sulphatising and when reducing, the electrolysis is carried out with a voltage of 2-2.5 volts. The current density is 500 amp/m. on t e surface of the lead powder, whilst that on the mercury surface is 1400 amp./ n1 The sulphation takes 10 to 12 hours. Under the above conditions, the current yield is 93-95% for the alkali and 88 to 90% for the acid. 1000 kg. of sodium hydroxide and 1150 kg. of sulphuric acid as well as hydrogen gas in a quantity equivalent to the alkali and oxygen gas in a quantity equivalent to the sulphuric acid, are produced with 3400-3600 kilowatt/hours.

With a series-arrangement of 41 cells, lead sulphation takes place in cells and sulphuric acid formation in 21 cells. A saturated sodium sulphate solution is fed into the system which contains 44% of sodium sulphate. The fiow speed of the sodium sulphate solution is so adjusted that the concentration thereof does not sink &

below 25% even after the twentieth cell. The lead electrode can thus be sulphated with a good yield.

Pure water is fed into the first cell of a series of cells arranged for a sulphuric acid cycle and the flow speed is so adjusted that the concentration of the sulphuric acid flowing out of the twenty-first cell is 40% at the utmost, since a sulphuric acid of higher concentration would disadvantageously influence the lifetime of the cell on account of its corrosive effect.

The lines connecting the individual electrolytic cells and decomposition cells, the mercury pump as well as the line which leads off hydrogen from the decomposition cell, have not been shown in the drawing.

I claim:

1. A process for the production of sulfuric acid and caustic soda by the electrolytic decomposition of a sodium sulfate first solution in an electrolytic cell containing lead and mercury active electrodes and a further electrode resistant to sulfuric acid, said process comprising the steps of:

(a) immersing said lead and mercury electrodes in said solution of sodium sulfate;

(12) arranging said lead active electrode as an anode and said mercury active electrode as a cathode and electrolyzing said solution, thereby forming sodium amalgam on said mercury electrode and lead sulfate on said lead electrode;

(c) withdrawing said solution of sodium sulfate from said cell and replacing same with a second solution of sulphuric acid sufiicient to bridge said lead electrode and said further electrode;

(d) arranging said lead electrode formed with said lead sulfate as a cathode and said further electrode as an anode, and electrolyzing said solution of sulfuric acid to reconvert said lead sulfate to lead and increase the sulfuric-acid concentration of said second solution; and

(e) decomposing said sodium amalgam in the presence of water to form sodium hydroxide.

2. A process according to claim 1 wherein said first and second solutions are each successively passed through a plurality of electrolytic cells whereby the sodium sulfate content of said first solution successively decreases and the sulfuric acid content of said second solution successively increases.

3. A process for the production of sulfuric acid and caustic soda by the electrolytic decomposition of a sodium sulfate first solution in an electrolytic cell containing lead and mercury active electrodes and a further electrode resistant to sulfuric acid, said process comprising the steps of:

(a) disposing said lead, further and mercury electrodes in vertically spaced ascending relationship within said cell;

(b) introducing said solution of sodium sulfate into said cell in suffieient quantity to bridge said active electrodes;

(0) arranging said lead active electrode as an anode and said mercury active electrode as a cathode and electrolyzing said solution, thereby forming sodium amalgam on said mercury electrode and lead sulfate on said lead electrode;

(d) withdrawing said solution of sodium sulfate from said cell and replacing same with a second solution of sulfuric acid sufficient to bridge said lead electrode and said further electrode but below the level of said mercury electrode;

(0) arranging said lead electrode formed with said lead sulfate as a cathode and said further electrode as an anode, and electrolyzing said solution of sulfuric acid to reconvert said lead sulfate to lead and increase the sulfuric-acid concentration of said second solution; and

(f) decomposing said sodium amalgam in the presence of water to form sodium hydroxide.

4. A process for the production of sulfuric acid and caustic soda by the electrolytic decomposition of a sodium sulfate first solution in an electrolytic cell containing lead and mercury active electrodes and a further electrode resistant to sulfuric acid, said process comprising the steps of:

(a) disposing said lead, further and mercury electrodes in vertically spaced ascending relationship within said cell;

(b) introducing said solution of sodium sulfate into said cell in sufficient quantity to bridge said active electrodes;

(0) arranging said lead active electrode as an anode and said mercury active electrode as a cathode and electrolyzing said solution, thereby forming sodium amalgam on said mercury electrode and lead sulfate on said lead electrode;

(a') withdrawing said solution of sodium sulfate from said cell and replacing same with a second solution of sulfuric acid sufi'icient to bridge said lead electrode and said further electrode but below the level of said mercury electrode;

(e) arranging said lead electrode formed with said lead sulfate as a cathode and said further electrode as an anode, and electrolyzing said solution of sulfuric acid to reconvert said lead sulfate to lead and increase the sulfuric-acid concentration of said second solution; and

(f) withdrawing the mercury forming said mercury electrode and containing said sodium amalgam from said cell and continuously decomposing said sodium amalgam in the presence of water to form sodium hydroxide.

5. Apparatus for the electrolytic decomposition of sodium sulfate, comprising housing means forming a chamber, a lead active electrode disposed in said chamber, a further electrode resistant to sulfuric acid vertically spaced from said lead electrode within said chamber, a mercury active electrode vertically spaced from said further electrode within said chamber, and means for rendering said lead electrode anodic and said mercury electrode cathodic for electrolysis of a sodium sulfate solution bridging said active electrodes and forming lead sulfate at said lead electrode and sodium amalgam at said mercury electrode and for rendering said lead electrode cathodic and said further electrode anodic for electrolysis of a sulfuric acid solution replacing said sodium sulfate solution and out of contact with said mercury electrode.

6. Apparatus for the electrolytic decomposition of sodium sulfate, comprising housing means forming a first and a second chamber, a lead active electrode disposed in said first chamber, a further electrode resistant to sulfuric acid vertically spaced from said lead electrode within said first chamber, a mercury active electrode vertically spaced from said further electrode within said first chamber, means for rendering said lead electrode anodic and said mercury electrode cathodic for electrolysis of a sodium sulfate solution bridging said active electrodes and forming lead sulfate at said lead electrode and sodium amalgam at said mercury electrode and for rendering said lead electrode cathodic and said further electrode anodic for electrolysis of a sulfuric acid solution replacing said sodium sulfate solution and out of contact with said mercury electrode, and means for conveying the mercury of said mercury electrode out of said first chamber into said second chamber for treatment with water to form sodium hydroxide.

7. Apparatus for the electrolytic decomposition of sodium sulfate, comprising housing means forming a chamber, a lead active electrode disposed in said chamber, a further electrode resistant to sulfuric acid vertically spaced from said lead electrode within said chamber, a mercury active electrode vertically spaced from said further electrode within said chamber the geometric surface area of said lead electrode being from 2 to 10 times larger than that of said mercury electrode. and means for rendering said lead electrode anodic and said mercury electrode cathodic for electrolysis of a sodium sulfate solution bridging said active electrodes and forming lead sulfate at said lead electrode and sodium amalgam at said mercury electrode and for rendering said lead electrode cathodic and said further electrode anodic for electrolysis of a sulfuric acid solution replacing said sodium sulfate solution and out of contact with said mercury electrode.

8. Apparatus for the electrolytic decomposition of sodium sulfate, comprising a plurality of cells, each of said cells including housing means forming a chamber, a lead active electrode disposed in said chamber, a further electrode resistant to sulfuric acid vertically spaced from said lead electrode within said chamber, a mercury active electrode vertically spaced from said further electrode within said chamber, and means for rendering said lead electrode anodic and said mercury electrode cathodic for electrolysis of a sodium sulfate solution bridging said active electrodes and forming lead sulfate at said lead electrode and sodium amalgam at said mercury electrode and for rendering said lead electrode cathodic and said further electrode anodic for electrolysis of a sulfuric acid solution replacing said sodium sulfate solution and out of contact with said mercury electrode; and means connect ing said cells for successive electrolysis of said solution, certain of said cells being assigned to the electrolysis of said sulfuric acid solution, and a lesser number of said cells being assigned to the electrolysis of said sodium sulfate solution.

References Cited in the file of this patent UNITED STATES PATENTS 1,981,498 Engelhardt et al Nov. 20, 1934 2,150,775 Messner Mar. 14, 1939 2,669,542 Dooley Feb. 16, 1954 2,744,864 Muller May 8, 1956 FOREIGN PATENTS 594,443 Great Britain Nov. 11, 1947 

1. A PROCESS FOR THE PRODUCTION OF SULFURIC ACID AND CAUSTIC SODA BY THE ELECTROLYTIC DECOMPOSITION OF A SODIUM SULFATE FIRST SOLUTION IN AN ELECTROLYTIC CELL CONTAINING LEAD AND MERCURY ACTIVE ELECTRODES AND A FURTHER ELECTRODE RESISTANT TO SULFURIC ACID, SAID PROCESS COMPRISING THE STEPS OF: (A) IMMERSING SAID LEAD AND MERCURY ELECTRODES IN SAID SOLUTION OF SODIUM SULFATE; (B) ARRANGING SAID LEAD ACTIVE ELECTRODE AS AN ANODE AND SAID MERCURY ACTIVE ELECTRODE AS A CATHODE AND ELECTROLYZING SAID SOLUTION, THEREBY FORMING SODIUM AMALGAM ON SAID MERCURY ELECTRODE AND LEAD SULFATE ON SAID LEAD ELECTRODE; 